Patients with morbid obesity are at an increased risk for cardiovascular and renal complications, which are not only linked to traditional cardiovascular risk factors. Thus, we evaluated (a) the prevalence of albuminuria in non-diabetic and diabetic morbidly obese patients and (b) the effect of weight loss following bariatric surgery.We included 1307 patients (77% women, mean age 40 ± 12 years, BMI 45.6 ± 6.6 kg/m) in a cross-sectional study. A subgroup (n = 318) was followed up for 2 years after bariatric surgery. Weight, cardiovascular risk markers and a 75-g glucose tolerance test were determined. Albuminuria was assessed by collecting 24-h urine on three consecutive days.In the cross-sectional study, the prevalence of microalbuminuria was 16.0% (n = 209), of macroalbuminuria 3.1% (n = 41). The chi-square for the association of albuminuria and diabetes was 31.937 (p < 0.001). Of all patients with albuminuria, 42.0% exhibited normal glucose tolerance. In a multivariate regression analysis, systolic blood pressure (beta = 0.236; p < 0.001), log fasting insulin (beta = 0.309; p < 0.001) and log 2-h postprandial insulin (beta = - 0.173; p = 0.033) were predictive risk factors for albuminuria. Longitudinally, albumin excretion decreased significantly from 11.1 (6.4, 18.4 mg/24 h) to 7.8 mg/24 h (4.9, 13.0 mg/24 h; p < 0.001). In the group with albuminuria preoperatively, albumin excretion decreased from 65.7 (38.2, 147.1 mg/24 h) to 13.5 mg/24 h (8.4, 36.8 mg/24 h; p < 0.001). After adjusting for age, sex and baseline albuminuria, patients with lower creatinine clearance showed a smaller decrease of albuminuria (beta = 0.117; p = 0.021).A substantial portion of patients with morbid obesity exhibits microalbuminuria, nearly half of those present with normal glucose tolerance. After weight loss, we found a significant decrease of albuminuria, potentially indicating or even contributing to the known reduction of cardiovascular mortality after bariatric surgery.